When you arrive at Bangkok's Suvarnabhumi International Airport and after you have past though immigration and colected your luggage proceed to the 2 floor which is demestic departures at the far right end of the departures. 

You will be going though the checkin and baggage check again.
When you book the domestic flight allow at least 2 hours to get though arrivals and passport control .
Suvarnabhumi Airport is very large and can take up to 20mins to walk from the arrival gate to get to the main terminal.